OPA690IDBVR datasheet(13/22 Pages) TI

13 / 22 page

OPA690

SBOS223A

www.ti.com

DC gain for this circuit is equal to RF. At high frequencies,

the DAC output capacitance will produce a zero in the noise

gain for the OPA690 that may cause peaking in the closed-

loop frequency response. CF is added across RF to compen-

sate for this noise gain peaking. To achieve a flat

transimpedance frequency response, the pole in the feed-

back network should be set to:

ππ

GBP

which will give a closed-loop transimpedance bandwidth

f–3dB, of approximately:

GBP

R C

–

=π

HIGH POWER LINE DRIVER

The large output swing capability of the OPA690 and its high

current capability allows it to drive a 50

Ω line with a peak-to-

peak signal up to 4Vp-p at the load, or 8Vp-p at the output

FIGURE 6. High Power Coax Line Driver.

FIGURE 5. DAC Transimpedance Amplifier.

of the amplifier using a single 12V supply. Figure 6 shows

such a circuit set for a gain of 8 to the output or 4 to the load.

The 5pF capacitor in the feedback loop provides added

bandwidth control for the signal path.

SINGLE-SUPPLY ACTIVE FILTERS

The high bandwidth provided by the OPA690, while operat-

ing on a single +5V supply, lends itself well to high-frequency

active filter designs. Again, the key additional requirement is

to establish the DC operating point of the signal near the

supply midpoint for highest dynamic range. Figure 7 shows

an example design of a 5MHz low-pass Butterworth filter

using the Sallen-Key topology.

Both the input signal and the gain setting resistor are AC-

coupled using 0.1

µF blocking capacitors (actually giving

bandpass response with the low-frequency pole set to 32kHz

for the component values shown). As discussed for Figure 2,

this allows the midpoint bias formed by the two 1.87k

Ω

resistors to appear at both the input and output pins. The

FIGURE 7. Single-Supply, High-Frequency Active Filter.

OPA690

High-Speed

DAC

O = IO RF

GBP

→ Gain Bandwidth

Product (Hz) for the OPA690

Ω

OPA690

Ω

0.1

µF

400

Ω

+12V

5pF

1Vp-p

Ω

Source

8Vp-p

4Vp-p

Ω

Load

OPA690

1.5k

Ω

432

Ω

137

Ω

500

Ω

1.87k

Ω

1.87k

Ω

+5V

DIS

0.1

µF

150pF

0.1

µF

100pF

5MHz, 2nd-Order

Butterworth Filter

Frequency (Hz)

5MHz, 2nd-Order Butterworth Filter Response

100k

10M

–5

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